Seed & Chemical
Winter cereals one more tool to fight weeds
Fall seeded crops act as one of the 'little hammers' that together can match the performance of the 'big hammer' - herbicide.
November 26, 2007 By Helen McMenamin
Winter wheat is often touted as a way to get a handle on weeds like wild oats,
but winter cereals are not a complete cure for any kind of weeds, says Bob Blackshaw,
weed scientist with Agriculture and Agri-Food Canada at Lethbridge. "Winter
cereals are a great weed management tool," he says. "They're very
competitive against spring germinating weeds, so they can give you an edge there.
"Weeds take advantage of any crop type you grow year after year. When
your main crops are seeded at the same time every year, weeds that germinate
at that time tend to become your worst weeds. In rotations dominated by spring
cereals, spring germinating weeds become a problem. Growing crops with different
seeding times will provide different ecological niches for weeds, so particular
species are less of a problem.
|Lethbridge trials are comparing various seeding dates and seed rates
for winter wheat.
"Winter cereals work as part of a whole system of management over five
to 10 years to reduce weed populations. In a single use, no crop works as well
as a herbicide. But, over time, winter wheat in a system may reduce the need
"Weeds like redroot pigweed, wild mustard and lamb's quarters, which germinate
quite late in spring, are generally not a problem in winter wheat. The crop
usually out-competes those weeds," says Blackshaw.
Alberta Agriculture cropping systems specialist, Rob Dunn, agrees. "Usually,
the only spring weeds that are a problem in winter cereals are early emerging
weeds like kochia and stinkweed," he says.
Overwintering fall germinating weeds can be a concern. Downy brome, stinkweed,
flixweed and stork's bill are best controlled in fall; at one time, stork's
bill was not thought to overwinter, but it does survive prairie winters and
often does very well the following year. Fall or early spring application of
a low cost herbicide can control winter or early germinating broadleaf weeds
"Take care if you're spraying when nights are cool," cautions Dunn.
"Phenoxy products, especially those with dicamba, can cause crop injury
under those conditions." Wild oats can be a problem in winter cereals.
"You need to walk your crop," says Blackshaw. "Because winter
cereals are more effective competitors than most crops, they give you an opportunity
to use a lower rate of herbicide, or you can expect a lot fewer escapes with
the regular rates. Second flushes are less often a problem in winter cereals.
By the time there's a second flush of spring weeds, winter wheat is heading
"If wild oats are an issue in winter cereals, I usually suspect the stand
is less competitive than we'd like to see," says Dunn. "Plant density
is probably low. The target population for the newer, shorter varieties of winter
wheat is 20 to 30 plants per square foot, depending on moisture. That's 90 to
100 pounds per acre, but seed size varies quite a bit, so use a rate calculator
(www.agric.gov.ab.ca). Seeding earlier in the planting window improves stand
Fall seeding is an ideal situation for fall germinating weeds, particularly
downy brome. At present, downy brome is found across the southern prairies.
It has been assumed to be a southern weed that cannot survive winter in more
northerly areas, but researchers at Saskatoon have found downy brome is at least
as winter-hardy as any winter wheat. Blackshaw expects it to spread to any area
where the cropping system allows it to gain a foothold. "Downy brome germinates
in fall or very early spring and it's very easily controlled with glyphosate,
so it's not a problem in spring crops," he says. "But, zero-till winter
crops are an ideal situation for it."
The traditional advice to broadcast most of a winter wheat crop's nitrogen
needs in very early spring may not be the best way to minimize weed pressure
or to maximize utilization of nitrogen.
"Repeated broadcasting of nitrogen increases weed pressure. I'm not sure
it has a great effect once in four years and it can be an efficient way to fertilize.
On the other hand, putting on all your nitrogen at seeding doesn't seem to reduce
stand density. I'd sooner see nitrogen banded in fall," says Blackshaw.
"A diverse crop rotation with seeding and spraying at different times
can lower weed pressure on crops. That allows you to insert herbicides when
they're needed. A variety of crops also allows you to choose different chemicals
with different modes of action, lowering the risk of herbicide-resistant weeds.
"Just changing the time of your pre-seeding burndown can affect the type
of weeds you have to spray out. Over a few years, weed populations drop,"